1. Field of the Invention
The present invention relates to a spectrum analysis technique of Rutherford backscattering spectrometry and the like, and more particularly, relates to a spectrum analyzer and a spectrum analysis method, accurately analyzing a composition distribution of a sample in the depth direction by measurement of an energy spectrum of scattered ions scattered from the sample which is composed of a single layer or multiple layers.
2. Description of the Related Art
Heretofore, in the fields of semiconductor techniques, crystal thin film techniques and the like, as a method for analyzing a sample such as a semiconductor device having a film structure composed of a single layer or multiple layers, various sample analysis methods such as Rutherford backscattering spectrometry (hereinafter referred to as “RBS method”) and Elastic recoil detection (hereinafter referred to as “ERD method”) have been widely used.
As a particular method for analyzing the concentration of a sample having an unknown composition distribution in the depth direction by using the above RBS method, a fitting method disclosed in Japanese Unexamined Patent Application Publication No. 2004-191222 may be mentioned by way of example. In this fitting method, after a theoretical energy spectrum (hereinafter referred to as “theoretical spectrum”) is calculated while an assumed composition distribution of a sample in the depth direction, which is assumed beforehand, is being changed, this calculated theoretical spectrum is compared with a measured energy spectrum (hereinafter referred to as “measured spectrum”) measured by a RBS analyzer using the RBS method described above, and the assumed distribution is searched in a trial and error manner so that the above two spectra coincide with each other, thereby determining the composition distribution of the sample in the depth direction. In addition to a sample having an unknown composition distribution, the analysis method described above may also be applied to a thickness analysis of a specific layer of a known sample.
However, in the fitting method described above, since the search of the assumed distribution is performed sensuously based on the knowledge, experience, and/or technique of a person conducting the analysis so that the theoretical spectrum and the measured spectrum coincide with each other, the analysis itself may depend on the personal capacity to a certain extent in many cases. Hence, due to individual differences, the analysis speed, analysis time, and analysis accuracy may vary.
Incidentally, in recent years, a high resolution sample -analyzer was developed which effectively analyzes the composition distribution of a sample in the depth direction without using the above fitting method, the method being disclosed in many papers such as Japanese Unexamined Patent Application Publication Nos. 2004-20459 and 2003-344319, and “High resolution RBS system” by Yoshikazu. Mori et al., Kobe Steel Technical Bulletin, R & D, Vol. 52, No. 2, pp. 53 to 56 issued on Sep. 1, 2002.
Referring to FIGS. 1 and 2, the structure and functions of a high resolution RBS analyzer (hereinafter referred to as “HRBS analyzer”) A, which is one example of the above high resolution sample analyzer, will be briefly described. FIG. 1 is a schematic view showing the entire structure of the HRBS analyzer A, and FIG. 2 is a schematic view showing individual constituent elements of the HRBS analyzer A.
As shown in FIGS. 1 and 2, the HRBS analyzer A described above has a basic structure composed of an ion beam generator X, a Wien filter Y provided with a slit 7 (see FIG. 2), a quadrupole lens 11, a vacuum container 3 in which a sample 2 to be analyzed is placed, and an electromagnetic spectrometer Z measuring an energy spectrum of scattered ions (one example of scattered particles) scattered from the surface of the sample 2. Reference numeral 21 shown in FIG. 1 indicates a goniometer which changes the angle of ion beams incident onto the sample by changing the position of the sample placed in the vacuum container 3, and reference numeral 22 indicates a transfer rod carrying the sample in and out of the vacuum container 3.
The ion beam generator X described above accelerates light ions, which are generated by an ion source 12 using a gas (such as a helium gas) supplied from a cylinder 15, by a high voltage supplied from a Cockcroft type high voltage circuit 14 so as to have a constant energy in an acceleration tube 13, and irradiation of the ions thus accelerated is then performed. The Wien filter Y and the slit 7 described above cooperatively extract specific ions (such as monovalent helium ions) from ion beams 1 which are accelerated and irradiated by the ion beam generator X. The Wien filter Y is a filter formed so that the deflection (proportional to the kinetic momentum of ions) caused by a magnetic field generated by a magnetic pole 17, a coil 18, and a return yoke 19 and the deflection (proportional to the energy of ions) caused by an electrical field generated by parallel electrodes 20 work in opposite directions to each other with respect to ions passing through the Wien filter Y. Hence, in the Wien filter Y described above, specific ions (such as helium monovalent ions) of the ion beams 1 are allowed to travel straight, and the trajectories of the other ions (such as divalent helium ions and hydrogen ions) are curved. By the above characteristics of the Wien filter Y, all the ions except the specific ions used for analysis cannot pass through the slit 7 and are eliminated, the slit 7 being provided at the downstream side of the Wien filter Y along the incident direction of the ion beams so as to only allow ions traveling straight to pass through the slit 7. As described above, the specific ions extracted from the ion beams 1 are converged by the quadrupole lens 11 provided at the downstream side of the slit 7 along the incident angle direction of the ion beams, and a predetermined beam spot is then irradiated onto the surface of the sample 2. The specific ions irradiated onto the surface of the sample 2 are scattered thereon, and some of the scattered ions are incident onto the electromagnetic spectrometer Z. This electromagnetic spectrometer Z deflects scattered ions passing therethrough by a magnetic field generated by a coil 4, a return yoke 5, and a magnetic pole 6 in accordance with the energy of the scattered ions and then guides them to a detector 8. The detector 8 is an energy sensitive type ion detector using a semiconductor such as silicon or germanium and has a plurality of channels. In this detector 8, when the positions (deflection by the magnetic field) of the channels to which the scattered ions are guided are detected, the detection signals are sent to a computer (or simulator) not shown in the figure, and the number of the scattered ion counts is measured. The positions and the number of the scattered ion counts thus obtained are used for calculating (measuring) an energy spectrum of the scattered ions scattered from the surface and the inside of the sample 2 by the above computer.
Next, referring to FIG. 3, the basic principle of a related analysis method will be described which analyzes the composition distribution such as the thickness of a sample based on a measured energy spectrum of the scattered ions. As shown in FIG. 3, the sample 2 has a two-layered thin film structure formed of a layer 21 composed of a heavy element P having mass number M1 and a layer 22 composed of a light element Q having mass number M2 provided in that order from the top. In this figure, reference numeral 23 (oblique line portion) indicates a silicon substrate holding the sample 2.
When an ion 30 (hereinafter referred to as “incident ion”) emitted from the ion beam generator X toward the sample 2 is irradiated onto the sample 2, followed by collision with component atoms (composition elements) of the sample 2, this collision may be substantially regarded as the elastic scattering. Hence, as shown in FIG. 3, for example, when the incident ion 30 having energy Eo and mass number m collides with the element P present at the surface of the layer 21 of the sample 2 and is then reflected by the elastic scattering in the direction of a scattering angle (detection angle) φ, energy Es of a scattered ion 30s thus reflected is given by equation (1) below based on the law of conservation of momentum.
                              E          1                =                  K          ·                      E            0                                              (        1        )                                K        =                              {                                                                                                      M                      1                      2                                        -                                                                  (                                                                              m                            ·                            sin                                                    ⁢                                                                                                          ⁢                          ϕ                                                )                                            2                                                                      +                                                      m                    ·                    cos                                    ⁢                                                                          ⁢                  ϕ                                                                              M                  1                                +                m                                      }                    2                                    (        2        )            
In the above equation, K is called a kinematic factor (hereinafter referred to as “K factor”) and is a coefficient representing the rate of energy loss at the collision of the incident ion 30. This K factor is represented as a function of mass number m of the incident ion 30 and mass number M1 of the collision ion P which collides with the ion 30, and from the energy Es of the scattered ion 30s, the mass number of the collision ion can be obtained.
In addition, when the incident ion 30 collides at a depth Δt apart from the surface of the layer 21 of the sample 2 and is then scattered, the energy of the ion 30 is attenuated by an electron cloud (free electron group traveling around the atomic nucleus of an element) while the incident ion 30 reaches the collision element and then jumps out of the surface of the sample after making the elastic collision. Hence, energy Ef of a scattered ion 30f reflected at the depth Δt and the energy Es of the scattered ion 30s scattered at the surface of the sample have the following relationship represented by equation (3).Es−Ef=ΔE   (3)
In addition, it has been known that ΔE is linearly proportional to the distance through which the ion travels, and when the attenuation coefficient (called stopping coefficient in some cases) of the energy of the incident ion 30 and the element number density of an element of the sample are represented by ε and N, respectively, ΔE of equation (3) can be represented by equation (4) below. The stopping coefficient ε is a function of energy E and charge Z of the sample. In addition, the stopping coefficient ε and the element number density N can both be treated as predefined physical quantities as long as the elements and the compositional structures of the layers 21 and 22 forming the sample 2 are known.ΔE=εNΔt   (4)
In FIG. 4, one example of an energy spectrum is shown which is obtained when ions having a predetermined energy are irradiated onto the sample 2 shown in FIG. 3. In FIG. 4, reference numeral Sp21 indicates a measured spectrum of the layer 21 of the sample 2, and reference numeral Sp22 indicates a measured spectrum of the layer 22.
In the measured spectra Sp21 and Sp22 thus measured, the widths of Sp21 and Sp22 each represent the energy attenuation ΔE. However, as shown in FIG. 4, since the measured spectrum has a Gauss distribution, the width of the measured spectrum cannot be easily specified. Hence, a spectrum width (a so-called half value width) at a half of the maximum value (H1 and H2 in FIG. 4) of the yield (the number of scattered ions counts detected by the detector) of the measured spectrum is regarded as the energy attenuation ΔE shown by a double sided arrow in the figure, and the thickness of an object layer to be analyzed is then obtained.
When a sample or a layer to be analyzed (hereinafter referred to as an “object sample” or “object layer”, respectively) is relatively thick, when the incident energy is high, or when the measurement is performed for a long period of time, a sufficient number of scattered ion counts can be measured. Accordingly, even when the above related analysis method is used, the thickness of the object layer or the composition of the object sample may be accurately calculated or analyzed when the half value width is regarded as the energy attenuation ΔE. However, when the object sample or the object layer is very thin, the incident energy is low, or the measurement is not allowed to be conducted for a long period of time, by the above HRBS analyzer A, a count value (spectrum) sufficient for highly accurate measurement cannot be obtained. Hence, in the case described above, when the related analysis method in which the layer thickness or the composition distribution is obtained based on the above half value width is performed, problems may arise in that the analysis result contains errors or varies. By the problem described above, when the object sample or the object layer is extremely thin, for the analysis of the thickness and/or the composition thereof, the fitting method described above must still be performed.
In addition, in the case of a multilayer structure, although depending on the scattering angle, the measured spectrum is obtained as an overlapping spectrum in which layer spectra of respective layers are overlapped with each other, that is, in which the spectra of the respective layers are not independently observed; hence, by the related analysis method described above, the spectrum of the object layer cannot be easily extracted, and as a result, there may be a problem in that the thickness and/or the composition of the object layer cannot be easily analyzed from the measured spectrum. This problem is also a problem which may probably occur in the analysis by the above fitting method.